Multilayer coatings are universally known in the industry. they are used for producing a lacquer structure which has various advantageous properties which cannot be obtained by using one coating agent only. In the car industry, for example, two-component stopping media or electrodeposition coating stopping media are used in order to protect the metal parts from corrosion. Fillers (primer/surfacers) are usually designed to compensate irregularities of the substrate and so obtain a smooth homogeneous surface for various kinds of substrate. They are also designed to protect the stopping medium from gravel or other mechanical damage. The top coatings normally improve the appearance of the substrate. They are single-layer or multi-layer covering (top) lacquer coatings. The multilayer top coatings comprising a pigmented base layer covered with a clear lacquer layer.
Multilayer lacquer coatings of this kind are described e.g. in EP-A-89 497. An aqueous anionic binder in a metallic base layer is applied to conventional substrates, followed by application of a normal conventional single-component (German abbreviation 1K) or two-component (2K) clear lacquer.
To improve the efficiency of a complex multilayer lacquer structure, the individual layers are usually optimised with a view to their special intended application. To obtain a good overall result, however, the successive layers must be well adapted to one another. For example, the layers must adhere very firmly to one another. More particularly, adhesion between the individual lacquer layers is required under mechanical stress, e.g., from stone chipping. Another problem, which is also connected with adhesion, is the resistance to "damp heat". It is known that under various damp storage conditions, moisture my accumulate at the interphase of individual layers multilayer systems. This results in loss of adhesion or blistering between the lacquer layers.
One possibility of improving the adhesion is to use adhesion mediators in the coating formulations. The primers can optionally diffuse into the surface later, depending on the chemical structure, and thus increase the adhesion to the next layer of lacquer. For example, DE-OS 39 32 744 describes the use of zirconaluminate compositions to improve adhesion. It is also known to use reactive adhesion mediators. These additives, however, have to be selected for each layer. Very often they have side-effects, e.g. tendency to form craters, which finally prevent a good lacquer structure being obtained.
EP-A-0 421 247 describes a process in which two electrochemically deposited lacquer layers for improving the optical properties are described. An anionic layer (ATL) is first deposited, then stoved, provided with a second layer in the form of a cathodic electrodeposition lacquer coating (KTL) and stoved. The reference mentions the corrosion resistance and the general appearance. However no subsequent coating layers are applied on the primer layers. In DE-OS 38 05 629 a stone chip resistant layer in the form of a coating agent based on an anionic binder is applied to commercial KTL. The covering layer is a commercial alkyd/melamine covering layer. The aim of this lacquer structure is to improve the protection against gravel given by aqueous anti-gravel coating agents, by using special resins. A conventional solvent-containing covering lacquer is used. The multiple coating has the usual weaknesses with regard to resistance to damp heat. All the anti-gravel layers in multilayer structures hitherto described consist of anionic binder systems.